Some influences of the development of higher plants upon the microorganisms in the soil: IV. Influence of proximity to roots on abundance and activity of microorganisms. V. Effects of plants upon distribution of nitrates

Starkey, R.L.

Soil Sci 32(5): 367-393, 395-404

1931


Accession: 025485345

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Abstract
IV. Studies were concerned with the development of microorganisms and biological activity in soil, about the roots of beets, beans, maize, and sweet clover at various periods during the growing season. The abundance of bacteria, actinomyces, and fungi was determined. The microorganisms were much more abundant about the roots, the most pronounced increases being noted in the region of the root surfaces where the organisms were many times as numerous as in the soil close to the roots. The general bacterial population and the bacteria of the Radiobacter group were affected by root growth to a much greater extent than the actinomyces and filamentous fungi. Greater numbers of bacteria were found on roots of legumes than on those of non-legumes, but the non-legumes exerted marked effects on the organisms. No characteristic effect of legumes was observed except in the region of the root surfaces. In general, the greater the distance from the region of extensive root development, the smaller the number of bacteria found in the soil. Effects of roots on fungi and actinomyces were noted only in material obtained from the root surfaces. Greater amounts of CO2 were produced by soils obtained from regions of extensive root development than from soils obtained at any distance farther removed from the plants. A close correlation appeared between the amounts of CO2 produced by the soils and the abundance of bacteria detected in the samples. Nitrification was most active in soils obtained from regions of maximum root development. An explanation of the relationships between soil microorganisms and plant roots is advanced.-V. Determinations of the abundance of nitrates in soils about roots of developing plants and in soils free from roots indicate that, under field conditions, plants greatly lower the nitrate content of soils. The degrees of removal of nitrates differ with the stages of growth of the plants. The nitrate contents of soils at different distances from the center of root development are different, greater amounts of nitrates being found at a distance from the absorption system of the plant. The modification of the nitrate content by plant development appears to be no factor of importance in bringing about acceleration in microbial activity which has been observed to accompany plant growth.